The present invention relates to an implant for the spine, and more particularly, but not exclusively relates to an anterior plate for maintaining a bone graft, bone anchorage screws for the plate and means for blocking the screws and preventing any migration of the latter.
Spinal plates may be introduced from the anterior to stabilize the cervical spine and maintain in position or secure a bone graft which fills the spaces left by the extraction of at least one vertebral disc and, possibly, part of a contiguous cervical vertebra.
The bone anchorage screws used for fixing this type of plate in position are either unicortical, and therefore short, since they pass through only the anterior cortical of the vertebra, or bicortical, and therefore long, since they extend through the anterior cortical and the posterior cortical. The short unicortical screws utilize a locking element when there is a possibility the screws may back-out. On the other hand, the long bicortical screws, while less likely to unscrew, have other limitations making them less desirable in certain situations.
In practice, the locking element employed up to the present time for unicortical screws is an additional screw which is positioned between two bone anchorage screws and whose head overlaps the heads of these two screws. While there are thin cervical plates without a locking element, the addition of an existing locking element to a given plate design typically results in an increase in plate size, such as the plate""s thickness. These additions may also result in a greater plate width than is desirable. Thus, the general desire to further decrease the size of surgical implants indicates a need for new plate and/or locking element arrangements.
Accordingly, one object of the present invention is to provide a unique device for stabilizing a patient""s spine.
Additionally or alternatively, another object of the invention is to provide an anterior cervical plate provided with antimigration means for the screws arranged in such manner as to avoid increasing the overall size of the plate.
One form of the present invention is a unique device for stabilizing a patient""s spine. In a further form, a slide is incorporated into a spinal plate that may be operable to serve as a locking element.
According to another form of the invention, a plate has means for blocking the bone anchorage screws that comprises at least one slide slidably mounted on the plate so as to be capable of partially covering at least one anchorage screw head, and the slide cooperates with means for retaining it on the head of the screw.
In still another form, the slide is formed by a thin platelet provided with at least one flange and slidable in a complementary cavity provided on the surface of the plate, the cavity having a ramp on which the flange can be engaged; the retaining means are formed by a boss on the plate which clicks into an associated recess when the slide is in its position for blocking and locking the screw. The cavity and the platelet may be so dimensioned that the surface of the platelet is flush with that of the plate when it is placed in its cavity. Consequently, for this form, the overall thickness of the anterior plate may not be increased by the slide.